The present invention relates to radio frequency (RF) receive coils for magnetic resonance scanners. The invention finds particular application in isolating or decoupling an RF receive coil during an imaging process. It is to be appreciated however, that the present invention may find further application in isolating individual coil portions of a single coil or in other arts in which selective RF signal reception is practiced.
Magnetic resonance imaging (MRI) machines apply a main magnetic field through an examination region. This strong field, typically denoted B0, acts to align the nuclei within a subject to be examined. In some MRI machines, the B0 field is horizontally oriented, and in others it is vertically oriented.
In both horizontally and vertically oriented systems, magnetic resonance is excited in the aligned nuclei by a relatively strong orthogonal RF field, typically denoted B1. The B1 field causes the aligned nuclei or spins to tip into an plane orthogonal to the static magnetic field B0. Over time, the spins realign themselves with the B0 field emitting relatively weak radio frequency (RF) resonance signals as they precess. This resonance is detected by RF coils tuned to the specific resonance frequency desired. These resonance signals are passed to image processing equipment to reconstruct the signals into an image representation for display on a video monitor.
Typically, the transmit RF signals are orders of magnitude larger than the magnetic resonance signals generated by the excited nuclei and detected by the RF receive coils. To protect the sensitive receiver equipment and the coils, the receive coils are typically decoupled or detuned during the transmit phase of an imaging procedure. Accordingly, it is known to decouple receive coils using semiconductor switches or PIN diodes in conjunction with LC circuitry using one of two principal variants, namely active decoupling and passive decoupling.
With active decoupling, during the transmit phase of an imaging operation a bias is applied to a PIN diode semiconductor switch in conjunction with an LC circuit to decouple or detune the coil. As technology has improved and the power of the transmit RF pulses has increased and increasingly higher bias currents on the switching diodes have been used to ensure the receiver coil remains decoupled. Unfortunately, these higher bias currents introduce magnetic field distortions in the B0 field close to the subject degrading the image obtained.
With passive decoupling, antiparallel diode semiconductor switches in conjunction with LC circuitry are also employed. In this method, antiparallel combinations of high speed switching diodes decouple the coil in response to the transmit pulse itself. In other words, when the antiparallel combination of diodes is exposed to the high power transmit signal, each diode conducts during its respective half cycle. This allows high currents, but not low currents, to see a parallel resonant LC circuit which decouples the coil. While this method employs no bias currents and eliminates the associated B0 field distortions, the coil is always decoupled during the RF transmit pulse and always coupled or active during receive.
The present invention contemplates an improved method and apparatus to decouple a receive coil which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance imaging system includes a main magnetic field generator which produces a B0 field through an imaging region and a gradient field generator which produces gradients across the B0 field. A radio frequency transmitter coil transmits RF pulses through the imaging region to excite nuclei and generate magnetic resonance signals to be reconstructed into human readable images. A first arrangement of receive coil elements is configured to decouple during the transmit phase and is configured to receive the magnetic resonance signals during the receive phase. A second arrangement of receive coil elements is configured to decouple during both the transmit and receive phases.
In accordance with another aspect of the present invention, an electrical path is provided which conducts received magnetic resonance signals away from the first arrangement of receive coil elements and conducts bias currents to at least the second arrangement of receive coil elements.
In accordance with another aspect of the present invention, the second arrangement of receive coil elements includes decoupler circuits which inhibit coil elements from resonating both when exposed to a determined RF transmit pulse and when exposed to a DC bias current.
In accordance with another aspect of the present invention, the decoupler circuits include a capacitive element selected to tune the coil elements to the magnetic resonance frequency spectrum. An inductive element is selectively switched into parallel with the capacitive element to parallel resonate at the resonance frequency spectrum.
In accordance with another aspect of the present invention, the inductive element is switched by a switch including an antiparallel combination of diodes in series with the inductive element. The diodes selected to alternate between a substantially conductive state in response to the presence of the RF pulse from a substantially non-conductive state.
In accordance with another aspect of the present invention, where the inductive element is switched by an antiparallel combination of diodes in series with the inductive element. Selected diodes being switched from a substantially non-conductive state to a substantially conductive state in response to an applied DC bias.
In accordance with another aspect of the present invention, the first arrangement of receive coil elements consists of a first RF receive coil, while the second arrangement of receive coil elements consists of a second RF receive coil.
In accordance with another embodiment of the present invention, a method of magnetic resonance imaging includes passively decoupling receive coil elements in response to a transmitted radio frequency pulse. Following the passively decoupling step magnetic resonance signals are received in selected receive coil elements and other receive coil elements are actively decoupled.
In accordance with another aspect of the present invention, the passively decoupling step includes alternately switching a pair of antiparallel diodes to a conductive state in response to the RF transmit pulse permitting an inductive element and a capacitive element to parallel resonate during the transmit pulse providing a high impedance across the coil elements.
In accordance with another aspect of the present invention, the actively decoupling step includes applying a DC bias current to diode switches to parallel resonate a capacitive element and an inductive element to provide a high impedance across the coil elements.
In accordance with another embodiment of the present invention, a radio receive coil includes a plurality of first switch circuits disposed about the coil. The first switch circuits being biased to passively decouple the coil in response to a transmit pulse. A control signal path is also provided through which a DC bias is applied to the coil to bias the first switch circuits to actively decouple the coil in response to the DC bias.
In accordance with another aspect of the present invention, the coil further includes a second switched circuit which is biased to provide an RF or AC short at an input of a preamp.
In accordance with another aspect of the present invention, the first switch circuits include an inductive element and a capacitive element selected to parallel resonate at a desired frequency. An antiparallel combination of diodes in series with the inductive element are selected to conduct in response to both a transmit pulse and the DC bias.
In accordance with another aspect of the present invention, the second switch circuits include a diode biased between conductive and non-conductive states by the DC bias.
In accordance with another aspect of the present invention, the control signal path includes an electrical path between a DC current source and a coil. The path further includes at least one DC blocking capacitor for routing the DC bias to the first and second switch circuits.
One advantage of the present invention resides in the ability to passively decouple an RF receive coil during a transmit phase of a magnetic resonance imaging event while actively decoupling the coil during a receive phase using the same decouplers.
Another advantage of the present invention resides in the use of low switching or bias currents to actively decouple the coil during the receive phase minimizing B0 distortion as compared to other actively decoupled coils.
Another advantage of the present invention resides in the ability to reconfigure a localized receive coil for different applications.
Another advantage of the present invention resides in the ability to permanently or semi-permanently install a receive coil within a couch of an MR system without causing interference with other localized coils.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.